1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter module mounted on a radio terminal such as a cellular phone or the like and distinguishing a high frequency signal, and in particular, relates to an impedance matching technology for matching with a transceiver IC (RF-IC).
2. Description of the Related Art
FIG. 8 shows a configuration example of a front-end unit of a quadband GSM type cellular phone. In this example, an antenna switch IC 2 is disposed directly below an antenna 1. A filter bank (4 systems) 300 embedding a plurality of receive SAW filters (hereinafter, referred to as an RxSAW filter) formed on the piezoelectric substrate is connected to an antenna switch 2 on the reception side, and a transmission side has a configuration such that a transmitting signal from a transceiver IC 4 as a high frequency transmission and reception device is inputted into an antenna switch 2 via a power amplifier 5.
Generally, in a radio terminal, an impedance matching means is provided for coupling circuit elements together so as to restrain a reflection phenomenon generated due to mismatch of impedance in a high frequency signal system (refer to Patent Document 1, for instance). In the example in FIG. 8, since the output and input impedance of a transceiver IC 4 and a receive SAW filter 3 (hereinafter, referred to as an RxSAW filter) are different, it is impossible to simply connect the RxSAW filter 3 and the transceiver IC 4 directly, and provision of an impedance matching circuit 6 configured with an inductance, a capacitor, and a resistor is required between the RxSAW filter 3 and the transceiver IC 4.
When realizing the impedance matching circuit 6 using discrete parts, much expense in terms of time and effort is needed for its adjustment, and it also requires increased man-hours for design on a portable terminal manufacturer side. In addition, a plurality of parts is generally necessary as the matching circuit parts, which raises material costs for the parts.
Then, a so-called filter bank module shown in a configuration example in FIG. 9 has come on the market. This filter bank module 7 mounts or forms the RxSAW filter 3 and the impedance matching circuit collectively on the module substrate or inside the module substrate, and the output impedance of the module is matched with the input impedance of the transceiver IC 4 so as to directly connect with the transceiver IC 4.
[Patent Document 1] Japanese Patent Application Laid-open No. Hei 09-270619
Since an impedance matching work on a portable terminal manufacturer side becomes unnecessary due to the appearance of module shown in FIG. 9 or the like, the man-hours for development thereof can be reduced. However, since the module shown in FIG. 9 has a structure in which plural pieces of impedance matching circuits composed of a plurality of passive components and Rx SAW filters 3 are mounted on a substrate, it has problems in that it has too many parts and a large variation in overall characteristics in general. Even when the impedance matching circuit is realized with inter-layered parts formed inside an LTCC (Low temperature co-fired ceramics) substrate, not with discrete parts, the variation in characteristics of the whole module causes a problem due to variation in characteristics of the parts.
In addition to those, as shown in FIG. 8 and FIG. 9, a conventional impedance matching circuit requires a plurality parts for the impedance matching circuit per one system, which brings increase in cost. This reason will be explained in detail with reference to FIG. 10 and FIGS. 11A, 11B, and 11C.
FIG. 10 is an example of an SAW filter having an unbalanced input port and a balanced output port. In the present example, the output impedance of the SAW filter 3 is assumed to be Z saw (=R saw+j X saw). the output impedance of a filter bank module is assumed to be Zm (=Rm+j Xm), and the input impedance of an RF-IC is assumed to be Zic (=Ric+j Xic). Here, if there exists a relation of Z saw=Zic* (complex conjugation), an impedance matching circuit is not necessary, and the energy transfer loss (reflection loss) between both can be minimum, but in an ordinary combination of the SAW filter 3 and the RF-IC 4, since it is extremely rare that both are in the above-described complex conjugation relation, and Z saw≠Zic (R saw≠Ric and X saw≠−Xic) in general, it is necessary to design and configure an impedance matching circuit having complicated characteristics.
Concretely, a manner of configuration of an impedance matching circuit in the case of Z saw≠Zic (R saw≠Ric and X saw≠−Xic) will be shown in FIGS. 11A, 11B, and 11C. FIG. 11A is a case of no matching element, FIG. 11B is a case of connecting an inductor to an output port 3a in parallel as a matching element, and FIG. 11C is a case of connecting a capacitor and an inductor in series and in parallel as matching elements respectively to the output port 3a. The drawings on the left side of the respective figures are Smith charts, those in the center are transit characteristics, and those on the right side are configurations of the filter bank modules.
The centers of the Smith charts in FIGS. 11A to 11C are considered to be matching impedance of the RF-IC, and portions shown by bold solid lines are Zm (impedances in the filter bank modules). The mark ● in the drawing is an impedance in the vicinity of the center frequency of the filter when no matching is performed, and matching is performed so that the mark ● is moved closer toward the vicinity of the center of the smith chart by selecting the constant and circuit configuration of a matching part. As clearly seen from FIGS. 11A to 11C, if the following equation R SAW≠Ric and X saw≠−Xic are established, it is necessary to adjust both Rm and Xm at the same time with a matching circuit, which means that at least two or more parts are required for matching.
As described above, since the cost of parts for the SAW filter which requires a plurality of parts for a matching circuit is high, and the degree of freedom in variation of each part is high due to the increased number of the parts, the problem of variation in overall characteristics of the module is likely to occur. Moreover, having too many parts gives rise to another problem of requiring many man-hours to adjust the characteristics to achieve the most suitable output impedance of the module.
The present invention has been made under these circumstances, and an object thereof is to provide a surface acoustic wave filter module which enables to reduce the man-hours needed for characteristic adjustment and a method of manufacturing the filter module.